Gravimetry

1. gravi – metric
(weighing - measure)
►Definition:
a precipitation method based on the
determination of weight of a substance of
known composition that is chemically related
to the analyte
►Analyte: the compound or species to be
analyzed in a sample
Gravimetric Analysis

2. Precipitation Analysis
1.) Principle
(i)
Reagent + Analyte Solid Product (collect and measure mass)
(ii) Desired Properties of Solid Product
 Should be very insoluble
 Easily filterable (i.e., large crystals)
 Very Pure
 Known and constant composition
Few precipitates have all of these properties, but in most cases
appropriate techniques can help optimize these qualities

3. ► Reaction:
aA + rR -----> AaRr ppt
where:
 a is no of moles of analyte A
 r is no of moles of reagent R
 AaRr is a pure, insoluble precipitate
which we can dry and weigh or ignite to
convert to something we can weigh
 ppt=precipitate

4. 7 Steps in Gravimetric
Analysis
► Dry and weigh sample
► Dissolve sample
► Add precipitating reagent in excess
► Coagulate precipitate usually by heating
► Filtration-separate ppt from mother liquor
► Wash precipitate (peptization)
► Dry and weigh to constant weight

5. Precipitation:
precipitating agent
sample
dissolved
components
►Dissolve sample
►Add ppt’ing
reagent
►Filter
►Dry
►Weigh

6. Suction Filtration
► Filter flask
► Buchner funnel
► Filter paper
► Glass frit
► Heavy-walled
rubber tubing
► Water aspirator

7. ► Mother liquor

8. • Conditions:
1. Must be a stoichiometric reaction.
2. A stable product; no oxidation, dehydration
or gelatinous precipitates.
3. Must avoid side reactions which result in
coprecipitates.

9.  The particle size of solids formed by precipitation varies
enormously.
 At one extreme are colloidal suspension, whose tiny particles
are invisible to the naked eye (10-7 to 10-4 cm in diameter).
 Colloidal particles show no tendency to settle from solution,
nor are they easily filtered.

11.  At the other extreme are particles with dimensions
on the order of tenths of millimeter or greater.
 The temporary dispersion of such particles in the
liquid phase is called a crystalline suspension.
 The particles of a crystalline suspension tend to
settle spontaneously and are readily filtered.

12.  Precipitates form in two ways, by nucleation and by
particle growth. The particle size of a freshly formed
precipitate is determined by which way is faster.
 In nucleation, a few ions, atoms, or molecules (perhaps
as few as four or five) come together to form a stable
solid.

13. Process of Crystal Growth:
(i) Two Phases in Crystal Growth
Nucleation – molecules in solution come together randomly and form
small aggregates
Particle growth – addition of molecules to a nucleus to form a crystal
Crystal Growth

14. Process of Crystal Growth:
Nucleation and Particle growth always compete for molecules/ions
being precipitated.
 If nucleation is faster than particle growth:
- a large number of small aggregates occur giving colloidal
suspensions
 If particle growth is faster than nucleation:
- only a few, large particles form giving pure crystals
Colloidal suspension Crystal formation
Want to
Convert to

15. Components of a Solution
 Solute is the substance being dissolved – powder
 Solvent is the dissolving agent – water
Solute
Solvent
Dissolve

16. Factors Affecting Solubility
– Temperature – Pressure
Solubility is the amount of solute that will dissolve in a
given amount of solvent at a given temperature or
pressure.

17. Factors Affecting the Rate of
Dissolution
– Stirring
– Temperature
– Particle size/Surface
area

18. 1. Crystal growth is independent of degree of
supersaturation
2. Nucleation increases with degree of supersaturation
Minimization of supersaturation will produce the largest
particles
a solution that contains more solute than the maximum amount of
solute that can dissolve at that temperature in a solvent

19. A saturated solution contains the maximum amount of a solute that
will dissolve in a given solvent at a specific temperature.
An unsaturated solution contains less solute than the
solvent has the capacity to dissolve at a specific
temperature.
A supersaturated solution contains more solute than is
present in a saturated solution at a specific temperature.
Sodium acetate crystals rapidly form when a seed crystal is
added to a supersaturated solution of sodium acetate.
12.1

20. Solubility

21. Process of Crystal Growth:
Rates of nucleation vs. particle growth depends on:
 Amount of precipitation solute present
 Described by a quantity known as the Relative Supersaturation (R)
If R is large,
 Large relative amount of solute in solution
 Favors nucleation and colloid formation
In gravimmetry based on precipitations, a small value of R (~1.0) is desired in
order to favor large crystal growth
S
)
S
Q
(
R 

S = Solubility
Q = actual concentration of solute added to solution
Colloidal Particle

22. Gravimetric Analysis
Precipitation Analysis
Process of Crystal Growth:
Methods for Minimizing R
1. Increase temperature of solution
- increases S
- increase amount of solute that can be in solution at
equilibrium
2. Add precipitating reagent (precipitant) slowly while
vigorously mixing solution
- avoids local high concentrations of solution
- avoid nucleation and colloid formation
3. Keep volume of solution large
- keep concentration of analyte and precipitating
reagent low
4. Control S through chemical means
- by adjusting pH
- adding complexing agents
- example: precipitation of Ca2+ with C2O4
2-

23. successful colloid precipitation:
1. Add precipitant slowly and in slight excess
2. Digest precipitate (Heat, stir)
crystalline precipitate
Similar to colloids:
1. Dilute solution
2. Slow precipitant addition
3. Elevated temperature
4. Heat unstirred

24. Gravimetric Analysis
• Filterability of Precipitates:
• Colloidal suspensions
– 10-7 to 10-4 cm diameter
– Normally remain suspended
– Very difficult to filter
• Crystalline suspensions
– > tenths of mm diameter
– Normally settle out spontaneously
– Readily filterable

26. Filterability:
(i) Want product to be large enough to collect on filter:
 Doesn’t clog filter
 Doesn’t pass through filter
(ii) Pure Crystals
Colloidal suspension
 Difficult to filter due to small size
 Tend to stay in solution indefinitely

27. Peptization
Peptization refers to the process by which a
coagulated colloid reverts to its original dispersed
state.
• When a coagulated colloid is washed, some of the
electrolyte responsible for its coagulation is
leached from the internal liquid in contact with the
solid particles.
• Removal of this electrolyte has the effect of
increasing the volume of the counter-ion layer.

28.  The repulsive forces responsible for the original
colloidal state are then reestablished, and
particles detach themselves from the coagulated
mass.
 The washings become cloudy as the freshly
dispersed particles pass through the filter.

29. Coprecipitation
Coprecipitation is the phenomenon in which
soluble compounds are removed from solution
during precipitate formation.
There are four types of coprecipitation:
i) surface adsorption, ii) mixed-crystal formation,
iii) occlusion, iv) mechanical entrapment

30. Surface Adsorption
• Adsorption is a common source of coprecipitation that is likely
to cause significant contamination of precipitates with large specific
surface areas, that is coagulated colloids.
• Coagulation of a colloid does not significantly decrease the
amount of adsorption because the coagulated solid still contains
large internal surface areas that remain exposed to the solvent.

31. Mixed-Crystal Formation
• For this exchange to occur, it is necessary that the two ions
have the same charge and that their sizes differ by no more
than about 5%.
• MgKPO4, in MgNH4PO4, SrSO4 in BaSO4, and MnS in
CdS.
• The extent of mixed-crystal contamination increases as the
ratio of contaminant to analyte concentration increases.

32. Mechanical Entrapment
• Mechanical entrapment occurs when crystals lie close together
during growth. Here, several crystals grow together and in so
doing trap a portion of the solution in a tiny pocket.

33. Gravimetric Analysis
Precipitation Analysis
(iii) Types of Impurities
 Impurities absorbed or trapped within pockets in the crystal
 Occlusion
 Impurities similar to the analyte or reagent
 Impurities placed in the crystal instead of analyte

34. Both occlusion and mechanical entrapment are at a minimum
when the rate of precipitate formation is low, that is, under
conditions of low supersaturation.

35. Gravimetric Analysis
Precipitation Analysis
Miscellaneous Notes on Precipitation:
Impurities are undesirable
 Change the chemical composition of the precipitate
 Causes errors in the analysis
Ways to Minimize Impurities
1. Keep R small
- large pure crystals decrease occlusions and adsorbed impurities
2. Digestion allowing precipitate to stand in mother liquor
(precipitating solution), usually while being heated
- promotes removal of impurities from crystal
- increases size of crystals
3. Wash precipitate, redissolve the precipitate in fresh solvent and
reprecipitate
- helps decrease all types of impurities
4. Add a masking agent to solution
- keeps impurities from precipitating, but not analyte
Mn2+ + 6CN-  Mn(CN)6
4-
Masking agent
Color  Impurity

37. Gravimetric Analysis
Precipitation Analysis
Washing Precipitates
 Precipitates from ionic compounds
- need electrolyte in wash solution
- keep precipitate from breaking up and redissolving (peptization)
 Electrolyte should be volatile
- removed by drying
- HNO3, HCl, NH4, NO3, etc.
 Example:
AgCl(s) should not be washed with H2O, instead wash with dilute HNO3
Drying/Igniting Precipitates
 Many precipitates contain varying amounts of H2O
- adsorbed from the air (i.e. hygroscopic)
 Precipitates are dried for accurate, stable mass measurements
 Precipitates are also ignited to convert to a given chemical form

38. Gravimetric Analysis
Precipitation Analysis
Homogeneous Precipitation:
Precipitating agent is generated directly in solution by means of a chemical
reaction.
 Ideal case for precipitations
- agent is generated uniformly throughout the solution
- excess are avoided
 Example:
As OH- is produced, pH gradually increases  precipitates a number of compounds (CaC2O4)

39. Gravimetric Analysis
Scope of Gravimetric Analysis
1.) Accurate
2.) Inexpensive
 Only major equipment is
balance
3.) Method is more tedious than
other approaches
 must carefully consider how to
minimize potential
interferences

40. ►% of analyte, % A
►%A = weight of analyte x 100
weight of sample

41. ► % A = weight of ppt x gravimetric factor (G.F.) x 100
weight of sample
► G.F. = a FW[analyte]
b FW[precipitate]
►G.F. = # gms of analyte per 1 gm ppt